#include "petscsys.h" PetscErrorCode PetscCommDuplicate(MPI_Comm comm_in,MPI_Comm *comm_out,PetscMPIInt *first_tag)Collective
| comm_out | - Output communicator. May be comm_in. | |
| first_tag | - Tag available that has not already been used with this communicator (you may pass in NULL if you do not need a tag) |
PETSc communicators are just regular MPI communicators that keep track of which tags have been used to prevent tag conflict. If you pass a non-PETSc communicator into a PETSc creation routine it will attach a private communicator for use in the objects communications. The internal MPI_Comm is used to perform all the MPI calls for PETSc, the outer MPI_Comm is a user level MPI_Comm that may be performing communication for the user or other library and so IS NOT used by PETSc.